Most cytotoxic drugs exhibit undesirable toxic side effects due to their lack of selective action toward the tissues or cells requiring therapeutic effect. Various approaches have been pursued to achieve the selective delivery of cytotoxic agents to a target cell type.
Using biological receptor ligands as carriers of drugs to target these drugs to the cells of interest can reduce toxic side-effects and greatly improve the efficiency of drug delivery. For example, International Patent Publication No. WO97/19954 discloses conjugates of an anthracycline cytotoxic agent such as doxorubicin with a peptide hormone such as LHRH, bombesin or somatostatin. The cytotoxic agent is covalently attached to the peptide via a linker of formula —C(O)—(CH2)n—C(O)—, n=0-7.
Similarly, European Patent Application No. EP1118336 discloses conjugates of somatostatin analogs, e.g., octreotide, lanreotide, and vapreotide, and a cytotoxic drug, such as paclitaxel, doxorubicin, or camptothecin, through a spacer, wherein the spacer is also indicated to have the structure: —C(O)—(CH2), —C(O)—, n=0-7.
U.S. Patent Application Publication No. 2002/0115596 discloses conjugates of cytotoxic agents and oligopeptides in which the amino acid sequences of the peptides are indicated to be cleaved preferentially by free prostate specific antigen. Such conjugates are said to be useful for the treatment of prostate cancer and benign prostatic hyperplasia.
U.S. Patent Application Publication No. 2003/0064984 discloses conjugates of cytotoxic analogs of CC-1065 and the duocarmycins with cleavable linker arms and a targeting agent such as an antibody or a peptide. The cytotoxic analogs are indicated to be released upon cleavage of the linker.
International Patent Application No. WO02/34237 discloses conjugates of active agents covalently attached directly to a polypeptide. The polypeptide is said to stabilize the active agent, e.g., in the stomach, through conformational protection.
There remains, however, a significant need for targeted cytotoxic drugs with improved properties in respect of targeting specificity, systemic toxicity, and pharmacokinetics.